Liquid chemical absorbance measurements using conventional spectrometers employ cuvettes of fixed path length for holding the sample of the analyzed. If the concentration or absorbance of the sample is too high, the instrument may not measure the correct absorbance value due to limited polychromatic light scattering from the diffraction grating in the instrument. If the absorbance is too low, the resolving capability of the instrument may be exceeded due to noise limitations established by the detector, amplifiers and/or source or too low a light level measured by the detector. For each instrument there is an optimum range of absorbance values which have the best resolution, lowest noise and best linearity. However, with the concentrations of liquids available and the fixed path length of light passing through the liquid, the absorbance does not always fall within the optimum range for the instrument. In addition, if the path length of the cell is too short, it may be difficult to get the liquid sample into the cell. It is also possible to have within a given spectrum two or more absorbance peaks located in different portions of the spectrum which cannot both be accurately measured because the cell path length has been optimized for the absorbance typical of only one of them. A single path length can only quantitate optimally one absorbance peak if there is a large difference is absorbance values between peaks.
Another problem which may arise in the course of measurement of certain types of fluids is the formation of a coating on the window of the spectrophotometry cell. Such a coating may substantially reduce the light transmission through the cell, resulting in a change in signal level which is confounded with changes in sample absorption in conventional spectrophotometers.
It is in general an objective of the invention to provide a new and improved apparatus and method for making absorbance measurements.
Another objective of the invention is to provide an apparatus and method of the above character which overcome the limitations, problems and disadvantages of the absorbance measuring instruments as discussed hereinabove.